In semi-arid regions, soil water availability is often more correlated with crop yields than any other soil or meteorological factor. Thus, quantification of soil water depletion by crops is important in estimating seasonal water use and evaluating alternative dryland cropping strategies, crop species, and varieties. The purpose of this study was to estimate soil water evapotranspiration (ET) for several crops and tillage systems under semiarid conditions in Bushland, Texas (USA) and Sebele, Botswana. Evapotranspiration was estimated using the water balance approach with a neutron moisture meter to determine changes in soil water storage. Precipitation was measured at the sites and runoff and drainage were assumed negligible. In Bushland, ET was estimated for cowpea [Vigna unguiculata (L.) Walp.], grain sorghum [Sorghum bicolor (L.) Moench], and sunflower (Halianthus annuus (L.)] under no tillage (NT) and stubble mulch tillage (SMT) systems in a clay loam (Torrertic Paleustoll). In Sebele, ET was estimated for cowpea and grain sorghum in a sandy loam (Typic Haplustalf) with disc (SD) cultivation, double disc (DD) cultivation, and mouldboard ploughing (MP). Soil water contents were measured with a neutron moisture meter (NMM) to a depth of 2.3 m in Bushland and 0.8 m in Sebele. Both probes were calibrated in-situ at each of the sites. In Texas, soil water depletion by sorghum and sunflower were similar and extended to soil depths greater than 2.0 m. However, soil water depletion for cowpea was limited to less than 1.5 m soil depth. In Sebele, soil water depletion by cowpea and sorghum were similar probably because of the greater rainfall received during the growing season (537 mm) as compared with Bushland (220 mm). Estimated ET for two growing seasons in Bushland averaged 178, 254, and 248 mm for cowpea, sorghum, and sunflower, respectively. In Sebele, seasonal ET averaged 296 and 307 mm for cowpea and sorghum, respectively. At both sites, seasonal ET was not significantly influenced by tillage systems. In Bushland, grain yield of cowpea and sorghum exhibited a positive linear relationship with ET. Despite the lack of tillage effects on ET, sorghum grain yield was significantly (P<0.05) greater for NT as compared with SMT in Bushland. This suggests greater crop transpiration under NT which was offset by reduced soil water evaporation. Cowpea grain yield also increased in response to ET, but was unaffected by tillage. In Sebele, sorghum grain yield ranged from 1700 to 2900 kg ha^-1 and was not significantly influenced by tillage treatments or ET. Other factors such as slow infiltration rates, small soil water holding capacity, limited rooting depth, or fertility status of this sandy loam soil was be limiting yield.

Keywords: soil water content, evapotranspiration, neutron probe meter, water availability, tillage.